1. Field of the Invention
This invention pertains to a composition which is suitable as a material to coupling optical components such as lenses without substantial loss of transmission. The material is particularly suited to formation of optical component couplings for mercury lamps (i-line) and deep ultraviolet optical projection and illumination systems and systems employing ultraviolet light sources such as mercury lamps. The invention pertains to the coupled optical components, as well as the composition itself.
2. Background of the Prior Art
Conventional optical projection systems frequently comprise a mirror plus a refractive assembly. The refractive array frequently will include two components, that must be optically coupled, to transmit light. Resolution of the image projected is of course, limited, by the wavelength of the light transmitted. Shorter and shorter wavelengths are sought, to improve resolution. Examples of one technology in need of such improved resolution are projection systems used for the integrated circuit fabrication and manufacture technology.
Conventional projection and illumination systems require the optical coupling together of disperate materials, such as SiO.sub.2 and CaF.sub.2, as well as LiF, to obtain desired performance. Separate components must be optically coupled, that is, they must be joined in a relationship such that the transmitted light can pass therebetween. Typically, in conventional apparatus, the optical components can be coupled with a conventional cement. However, all known optical lens cements are either opaque to deep ultraviolet transmission, e.g., 240 nm, and similar wavelengths, or become opaque, or turn black, upon repeated exposure to ultraviolet transmission.
It should be further noted that the lenses cannot simply be physically pressed together, as perfect matching of surfaces never occurs, giving undesirable variable transmission between the surfaces. It should be further noted that frequently the lenses have differing coefficients of thermal expansion. In a combination of a SiO.sub.2 lens, and CaF.sub.2 lens, the coefficient of expansion of the calcium fluoride lens may be 20 times that of the silicon dioxide. Obviously, if these two materials were rigidly cemented together, a relatively small temperature change could cause one of them to shatter or at least induce birefringence that could impair image quality. Accordingly, a soft optical agent that allows differential expansion coupling between dissimilar materials yet provides a uniform low reflectivity lightpath is highly desirable.
Certain types of lens coupling media are known. Typically they will include a silicon oil. A typical silicon oil is available from Dow Corning Corporation, and marketed under the name DOW CORNING 200. This oil is a low viscosity fluid, insoluble in water, having a specific gravity at 25.degree. C. of about 0.98. Although substantially transmissive to deep ultraviolet and i-line (365 nm) light, the silicon oil's low viscosity makes it difficult to handle. It tends to smear and spread over the lens, the coupling interface, and everywhere else, and presents serious challenges to containment. Those products on the market incorporating such a silicon oil as the coupling composition for two optical lenses have been plagued by containment problems. Of course, any loss of optical coupling, disturbing the optical transmission, is intolerable. As the layer of coupling material is extremely thin, e.g., 0.02 mm or so, there is not substantial room for error.
Accordingly, it remains an object of those skilled in the art to obtain a composition suitable for coupling optical lenses, which has the transmissive characteristics necessary for deep ultraviolet and i-line wavelengths, and related high transmission at those wavelengths, is sufficiently fluid so as to provide for thin layer coatings between the lenses, and does not produce a large difference in index of refraction, such that too much of the image light is lost in reflection.
An additional very desirable property of the optical coupling medium is the ease with which it can be confined to the desired interface. An immobile gel is much preferred over a high surface energy oil which tends to leak, creep and migrate to places where it can seriously interfere with performance.